Until 1950s, uncrosslinked type polyethylene resin was mainly used for insulation of a power cable, but it was problematic in long-term heat resistance and durability. Since the development of curing technique for improving long-term heat resistance and durability of polyethylene by Union Carbide (USA) in the 1950s, power cables are mostly prepared with crosslinked polyethylene.
Polyethylene is crosslinked by a chemical reaction using an organic peroxide or silane (U.S. Pat. No. 6,284,178) or by the use of an electron beam (U.S. Pat. No. 4,426,497). Currently, in the cable industry, crosslinked polyethylene is prepared mostly by crosslinking using an organic peroxide.
Since crosslinked polyethylene resin is thermoset resin, it has superior heat resistance and chemical resistance as well as good electrical properties.
However, crosslinked polyethylene resin causes environmental pollution because the thermoset resin is unrecyclable. Thus, there is a need of environment-friendly un-crosslinked type thermoplastic polyethylene resin. But, because of noticeably worse heat resistance, it is restricted to be used for insulation of power cables.
Nevertheless, in some European countries including France, thermoplastic polyethylene resin is used for insulation of power cables to avoid the environmental problem of the crosslinked polyethylene resin.
In production of power cables with polyethylene crosslinked by an organic peroxide, the process of crosslinking is essential. The crosslinking process requires a high-pressure, high-temperature condition and has very low productivity. Even a slight change in the process condition may result in degraded product uniformity because of nonuniform crosslinkage.
During the crosslinking process, an organic peroxide is decomposed by heat and radicals which carry out crosslinking are produced therefrom. In this process, cumyl alcohol, methane, or the like are produced as byproduct and form bubbles in the insulator. To remove them, a high pressure of at least 5 atm should be applied. If left unremoved, the bubbles may result in breakage of the insulator.
Since Korea is surrounded by the sea on three sides, the air contains a lot of salts, which often erode the insulator of overhead power cables and result in fire. The erosion of the power cable insulator is called failure by tracking. It is an intrinsic phenomenon of the organic insulator whereby conducting paths formed on the surface of the insulator by the carbonization product results in surface dielectric breakdown. Although it is triggered by heat resulting from surface discharge or scintillation discharge, various causes are associated with its generation, including moisture, salts, acid rain, inorganic or fibrous dusts, chemicals, or the like.